How AHAs and BHAs Actually Differ in Exfoliation

Alpha hydroxy acids (AHAs) and beta hydroxy acids (BHAs) are both chemical exfoliants, but the term "chemical exfoliant" covers two mechanistically distinct processes. Where a physical exfoliant removes dead cells through abrasive friction, both AHAs and BHAs work by interrupting the molecular bonds that hold corneocytes — the flattened, dead cells of the stratum corneum — in place. The resemblance ends there.

The structural difference between the two acid classes is not cosmetic. AHAs are water-soluble compounds; BHAs are oil-soluble. That single chemical property determines where each acid travels within the skin, how deeply it acts, and which tissue environments it can reach. Understanding the distinction requires examining what each molecule actually does at the cellular and follicular level.

How Each Acid Type Breaks Down Cellular Cohesion

The stratum corneum is held together by protein structures called corneodesmosomes — molecular rivets that link adjacent corneocytes. Enzymatic activity normally degrades these structures over time, allowing cells to shed naturally in a process called desquamation. Both AHAs and BHAs accelerate this process by lowering the local pH, which activates those same degradative enzymes and directly weakens the ionic interactions holding the desmosomes together.

AHAs — a class that includes glycolic acid, lactic acid, mandelic acid, and citric acid — act primarily at the outermost surface of the stratum corneum. Because they are water-soluble, they distribute through the aqueous environment of the skin's surface and the hydrophilic spaces between dead cells. Glycolic acid, the smallest AHA by molecular weight, penetrates the stratum corneum most readily among the class, reaching slightly deeper layers of dead skin. Lactic acid, with a larger molecular size, acts more superficially. In both cases, the mechanism is the same: acid-catalyzed disruption of corneodesmosomes, leading to accelerated shedding of the surface cell layer.

BHAs — most commonly salicylic acid, a compound derived from willow bark — behave differently because of their lipid solubility. The follicular canal and the sebaceous material within it are lipid-rich environments. Salicylic acid's oil-soluble structure allows it to partition into this lipid phase and travel down the follicle, where it exerts its keratolytic and comedolytic effects. As detailed in the mechanism of how salicylic acid actually penetrates a pore, the molecule disperses through sebum, softening the keratinous plug that forms a comedone from the inside. This is a fundamentally different path than AHAs take — one that is simply unavailable to water-soluble compounds.

Salicylic acid also has mild anti-inflammatory properties independent of its exfoliating action. It inhibits the synthesis of prostaglandins, lipid signaling molecules involved in inflammatory cascades. This property is a consequence of its chemical relationship to aspirin (acetylsalicylic acid), not a feature shared by AHAs.

The Molecular Players: Acid Class, Concentration, and pH

Glycolic acid is the smallest AHA, with a molecular weight of 76 g/mol. Its small size correlates with faster and deeper penetration through the stratum corneum relative to other AHAs. It is derived from sugarcane and is among the most extensively studied hydroxy acids in dermatological literature.

Lactic acid has a larger molecular weight (90 g/mol) and penetrates more slowly, which is associated with a comparatively milder surface action. It also functions as a humectant at lower concentrations, attracting water into the stratum corneum. This dual behavior — exfoliant and hydration-supporting agent — is a property not shared by glycolic acid to the same degree.

Mandelic acid, derived from bitter almonds, is the largest of the commonly used AHAs. Its slower absorption rate is attributed to its aromatic ring structure, which increases molecular bulk. It is sometimes used in formulations where a slower onset of action is desirable.

Salicylic acid, the primary BHA in skincare formulations, carries a hydroxyl group on a benzene ring attached to a carboxylic acid group. This aromatic structure is what makes it lipophilic. At concentrations typically found in over-the-counter products (0.5–2%), it functions as a keratolytic agent — softening and loosening the bonds between keratinized cells — and as a comedolytic agent within follicles.

pH and free acid concentration govern the activity of both acid types. The undissociated (protonated) form of each acid is the active species — the form that can penetrate cell membranes and disrupt protein bonds. A formulation's pH determines what proportion of the acid remains undissociated. At a pH above 4, a progressively larger fraction of AHA molecules donate their proton and become ionized, reducing the available active species. Most effective AHA and BHA formulations are buffered to a pH between 3 and 4 to maintain meaningful concentrations of the active form. The integrity of the lipid barrier also influences how deeply either acid penetrates: a compromised barrier allows greater and less controlled acid diffusion into viable skin layers.

Where the AHA–BHA Distinction Breaks Down in Practice

The clean conceptual separation between "surface-acting AHAs" and "pore-penetrating BHAs" is real at the molecular level but becomes less tidy in formulation. Concentration, vehicle composition, and the condition of the stratum corneum all introduce variability.

High-concentration glycolic acid formulations — such as those used in professional peeling procedures — do penetrate into the viable epidermis, reaching the stratum granulosum and even the stratum spinosum. At these depths, the acid is no longer simply accelerating desquamation; it is disrupting living keratinocytes. The distinction between a surface exfoliant and a deeper chemical injury becomes a matter of degree, not kind. Consumer-available formulations are generally well below these concentrations, but the principle holds: depth of action is a function of concentration, pH, and contact time, not solely of acid class.

Lactic acid's humectant effect also introduces a complication. At low concentrations, it may increase surface hydration without producing meaningful exfoliation. A product labeled as containing lactic acid may therefore behave more like a moisturizing toner than an exfoliant, depending on its formulation pH and acid percentage. The label alone does not resolve this ambiguity.

Salicylic acid's lipid solubility is sometimes described as making it universally "better for oily skin," but this framing oversimplifies the mechanism. The acid's follicular penetration is a function of the lipid content of the follicular canal; in follicles with minimal sebum, the lipid pathway is less available, and the acid behaves more like a surface keratolytic. The oil-solubility advantage is most pronounced where sebum production is high enough to provide a meaningful lipid channel.

Combining AHAs and BHAs in a single formulation is chemically straightforward — there is no reactivity between the two acid classes — but the resulting pH environment must still support the undissociated form of both acids. A formulation that adequately activates one may not be optimally buffered for the other, and the interaction between the two exfoliating mechanisms is not simply additive.

What a Product Label Reveals — and Omits — About Acid Activity

In the United States, AHAs and BHAs in rinse-off products at concentrations above certain thresholds are regulated by the FDA as cosmetic ingredients subject to safety guidelines, though they are not classified as OTC drugs unless they carry a drug claim (such as acne treatment for salicylic acid). The FDA has issued guidance noting that AHA-containing products may increase UV sensitivity — specifically, photosensitivity of the skin — a consequence of the accelerated removal of the stratum corneum's outermost protective cell layers. This guidance is required to appear on AHA product labels as a sun-sensitivity warning.

A product label listing "glycolic acid 10%" conveys the total acid concentration but does not specify the free acid concentration — the fraction actually available in undissociated, active form. Without knowing the formulation's pH, the stated percentage provides an incomplete picture of likely activity. Two products with identical percentage claims but different pH values can have meaningfully different effective concentrations of active acid.

Salicylic acid in acne-treatment products is regulated as an OTC drug under FDA monograph conditions. The monograph specifies permitted concentrations (0.5–2%) and requires specific labeling language. A product carrying an acne-treatment claim with salicylic acid as the active ingredient has been formulated and labeled under these regulatory parameters. Products that list salicylic acid as a cosmetic ingredient without an acne claim are not subject to the same monograph requirements, meaning the regulatory context of the same molecule differs based on the claim the manufacturer makes.

Neither AHA nor BHA labels are required to disclose pH, buffering agents, or free acid percentage. The information most directly relevant to predicting a formulation's exfoliating activity is therefore largely absent from consumer-facing labeling.

AHAs and BHAs share a mechanism — acid-catalyzed disruption of the bonds that hold dead skin cells together — but their chemistry routes them to different tissue environments, and that routing is determined by a single property: water solubility versus oil solubility. The distinction is structural, not a matter of marketing emphasis.

Note: This explains how skincare ingredients and products work. It is not a skincare routine, it does not diagnose or treat a skin condition, and it is not a substitute for a dermatologist. Check the cited sources for current guidance.